Cathodic corrosion protection powder coating composition and method

ABSTRACT

The present invention provides a curable cathodic corrosion protection powder coating, which comprises a thermosetting resin, a zinc borate compound, a curing agent in an amount effective to cure the coating. Further, the present invention also provides a method of cathode corrosion protection which includes the steps of subjecting the substrate to a mechanical treatment, applying to said treated steel surface, the cathodic protective coating, and polarizing the coated material as a cathode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a cathodic corrosion protectioncomposition and method. In particular, this invention is directed to acurable powder coating composition comprising zinc borate and a methodfor applying the coating composition, which when applied onto a steel,or other ferrous substrate, provides an anticorrosive coating, effectivefor improving resistance to cathodic disbondment.

2. Description of the Related Art

One means for preventing corrosion of steel materials, in humidconditions containing electrolytes such as brine and salt solution iscathodic protection. Cathodic protection prevents dissolution of steelby maintaining a steel material as a cathode and inhibiting ionizationof iron. However, when the iron portion has a large area, consumption ofpower and a sacrificial anode increases. Therefore, the steel materialis not generally used directly for a cathodic protection, but in mostcases, a cathodic protection is effected in combination with an organiccoating and/or lining. Through this approach, major proportions of thesteel material are protected from corrosion by the organic coating, anddefective portions occurring in this organic coating such as scratchesand pin-holes, can be supplemented by cathodic protection.

In cathodic protection, metal ions are reduced and become insoluble onthe metal surface polarized to the cathode. Therefore, a satisfactoryeffect can be obtained by effecting anticorrosion by applying power incorrespondence with the metal ions which are to be dissolved. The amountof the metal ions to be dissolved is proportional to the surface area ofthe metal, and corresponds to the surface area of the defective portionsin the case of the coated steel material. However, it is extremelydifficult to know the exact surface area of the defective portions. Forthis reason, a cathodic protection is generally applied in excess.However, excessive polarization generates hydroxyl ions due tohydrolysis of water at the cathode, so that scratched portions of theorganic coating function as a cathode and are always exposed to analkaline atmosphere. When such a condition occurs, degradation of theorganic coating's adhesion points occurs on any of the interfacesbetween the steel material and the organic coating and between theorganic coatings, particularly at the portions at which alkaliresistance is weak, and cathodic disbanding of the organic coatingoccurs.

As a means for restricting such cathodic disbonding, a method ofcarrying out a chromate treatment or applying a zinc-rich primer coatingof a specific thermosetting epoxide resin has been proposed (seeJapanese Unexamined Patent Publication (Kokai) No. 59-222275). However,this technique is not sufficient to satisfy the high-level requirementfor the cathodic disbonding resistance in recent years, and does notemploy a zinc borate based pigment component in the composition.Japanese Unexamined Patent Publication (Kokai) No. 55-142063 discloses acomposition consisting of a polyvinyl butyral resin, a liquid epoxideresin, a borate compound, an epoxy-silane coupling agent and phosphoricacid as a pre-treatment composition for baking type. However, thiscoating composition is directed to a wash primer for pre-treating ametal and is different from the object of the present invention, and theresins used in this reference do not use a curing agent and arethermoplastic resins.

Methods of restricting such cathodic disbanding are generally known.Unfortunately, many of these techniques are not sufficient to satisfythe high-level requirement for the cathodic disbonding resistance inrecent years. A method of providing cathodic protection from corrosionby carrying out the steps of steel pre-treatment, applying a zinc-richthermosetting epoxide resin based powder coating, and subsequentlypolarizing the coated steel material as cathode, has been described inEuropean Patent EP 0 588 318 B1, Kaga. However, this technique islimited to coatings with relatively high levels (5-75 wt. %) of zinccompounds, which presents issues of solubility over long periods oftime, as well as the increased cost for the zinc borate compound.

Therefore, there is a need for coating compositions, preferably powdercoating composition, and methods of application thereof, which provideoptimum long term and high temperature and humidity cathodic disbondmentprotection, at a lower cost.

SUMMARY OF THE INVENTION

The present invention provides a curable cathodic corrosion protectioncoating, preferably a powder coating, in which the coating comprises:

-   -   (a) a thermosetting resin, or mixture of thermosetting resins;    -   (b) from about 0.5 to 4.75% by weight, based upon total solids        weight, of a zinc borate compound; and    -   (c) a curing agent(s) in an amount effective to cure the        coating.

The present invention further provides a method of cathodic corrosionprotection which includes the steps of subjecting the substrate to amechanical treatment, applying to said treated steel surface thecathodic protective coating, and polarizing the coated material as acathode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the cathodic disbondment resistancecorrelation with zinc borate concentration.

DETAILED DESCRIPTION OF THE INVENTION

To improve coating resistance to cathodic disbanding, the historicalapproach is to improve the ability to adhere to the steel by addingpolar groups, as described in U.S. Pat. No. 4,330,644 issued to Allen.Another approach is to add high levels of zinc borate to minimizecorrosion as described in Euro. Pat. No. 0 588 318 B1 issued to Kaga,which was previously mentioned.

The present invention is based upon the discovery that incorporation oflow levels (additive quantities) of a zinc borate compound into athermosetting resin based coating system, preferably a powder coatingsystem, and applying this finish to a steel substrate, provides acoating which has excellent resistance to cathodic disbanding,especially in long term high temperature and humidity conditions. Thecoating, and method of use thereof, of the present invention is usefulas a coating for steel substrates, including for example, but notlimited to, the internal and external surfaces of steel pipes,structural steel used in concrete, storage tanks, structural steel inmarine environments, and oil production tubing and casings.

In the coating composition of the present invention, any thermosettingresin can be used so long as it can firmly adhere to a steel material orto a steel material subjected to a mechanical treatment such as blastcleaning or to a steel material subjected to a chemical treatment suchas a chromate treatment or treatment with zinc phosphate. Examples ofsuch resins include an epoxy resin with an epoxy resin curing agent, apolyol resin with isocyanates, an acryl modified epoxy resin with apolymerization initiator, an alkyd resin, a humidity curing urethaneresin, and so forth. Preferably the thermosetting resin is an epoxyresin. More preferably the thermosetting resin is an epoxy resin, ormixtures of epoxy resins, used in conjunction with an effective epoxycuring agent.

The coating composition of the present invention preferably containsabout 25 to 90% by weight, based upon total solids weight, of athermosetting resin, or any mixture of thermosetting resins. Morepreferably, the composition contains about 60 to 80% by weight, basedupon total solids weight, thermosetting resin, or mixtures thereof.

Examples of epoxy thermosetting resins suitable for the presentinvention are di-glycidyl ethers of 4,4-(bishydroxyphenyl) alkanesprepared by reacting 4,4′-(bishydroxyphenyl) alkanes such as bisphenolA, bisphenol F, bisphenol AD, etc., with epihalohydrin. There is noproblem in using glycyldyl ethers of 4,4-(bishydroxyphenyl) alkanes asthe principal component in combination with a phenol novolac epoxy resinor cresol novolac epoxy resin, or other multifunctional resins. Epoxyresins of this kind are commercially available on the market as “EPON”and “EPIKOTE” (both are products of Resolution Performance Products,LLC.), “EPOTOHTO” (a product of Tohto Kasei K.K.), “ARALDITE” (a productof Vantico), “EPICLON” (a product of Dainippon Ink & Chemicals, Inc.),“Dow Epoxy” (a product of Dow Chemical International, Ltd.), and soforth. A particularly useful epoxy is “EPON” 2024 bisphenolA/epichlorohydrin thermosetting epoxy resin, available from ofResolution Performance Products, LLC.

The coating composition of the present invention also contains a curingagent, or mixture of curing agents, incorporated in an amount effectiveto cure the coating. Preferably the coating contains about 1 to 35% byweight, based upon total solids weight, of a curing agent, or anymixture of curing agents. More preferably, the composition containsabout 2 to 20% by weight, based upon total solids weight of a curingagent, or mixtures thereof.

Conventional curing agents for epoxy resins containing a plurality ofaddition-polymerizable functional groups to the epoxy group of an epoxyresin in the molecules thereof can be used as the epoxy curing agent.Examples of the epoxy curing agents of this kind include diamines suchas aliphatic diamines, aromatic diamine and heterocyclic diamines,various modified products of these diamines, polyamide resins obtainedby the reaction with aliphatic acids and their dimers, acid anhydrides,thiols, phenols, and so forth. These curing agents are commerciallyavailable on the market as “EPOMATE” (a product of ResolutionPerformance Products, LLC.; various heterocyclic diamine modifiedproducts), “SUMMIDE” (a product of Sanwa Chemical Industry Co., Ltd.;various amine adducts or polyamides), “TOHMIDE” (a product of FujiKasei-Kogyo K.K.; various polyamides), “EPIKURE” (a product line ofResolution Performance Products, LLC.; various amine adducts, thiols,phenols), “RIKASHIDE” (a product of New Japan Chemical Co., Ltd.; acidanhydrides), and so forth. Instead of being the addition polymerizationtype, the curing agents may be accelerated dicyandiamides havingaddition reactivity and self-polyaddition catalytic activity betweenepoxy groups, the derivatives thereof, and imidazoles. These epoxy resincuring agents are appropriately selected and used in accordance with thetypes of the coating, the curing conditions (ordinary temperaturecuring, heat curing, etc.), and so forth. A particularly useful epoxycuring agent is “EPIKURE” P104, an accelerated dicyandiamide, availablefrom Resolution Performance Products, LLC.

In cases where the thermosetting curing system comprises a polyol resinand an isocyanate curing agent, urethane bonds, also known as carbamatebonds, are formed by reacting the hydroxyl groups of the polyol resinwith the isocyanate groups. Examples of the polyol resin are polyolresins obtained by conventional preparation methods, such as polyesterpolyols, acrylic polyols, polyether polyols, etc., and these polyolresins are used either alone or in mixture of two or more polyols. Thecuring components of these polyol resins are commercially available onthe market as polyol diisocyanate adducts prepared by adding anequimolar diisocyanate to the number of hydroxyl groups of polyhydricalcohol compound and diisocyanate polymers obtained throughself-polyaddition by reacting water with diisocyanates.

When the thermosetting resin comprises an acryl modified epoxy resin,such resin is formed by introducing a polymerizable double bond into theepoxy group of the epoxy resin by the addition reaction of acrylic acid,and the epoxy resins having various grades are commercially available.These acryl modified epoxy resins can be polymerized by radicalpolymerization, and can be cured by the use of a catalyst such as anorganic peroxide, a photo-polymerization initiator, etc., as thepolymerization initiator.

In the present invention, if an alkyd resin based thermosetting systemis used, the alkyd resin can be obtained by dehydration polycondensationor addition polymerization of polyhydric alcohols, polyvalent carboxylicacid or their anhydrides by aliphatic acids. Examples of the polyhydricalcohols as the starting material of the alkyd resin includes dihydricalcohols such as ethylene glycol, propylene glycol, 1,3-butylene glycol,1,6-hexane diol, diethylene glycol, dipropylene glycol, neopentylglycol, triethylene glycol, hydrogenated bis-phenol A, bis-phenoldihydroxypropyl ether, etc.; trihydric alcohols such as glycerin,trimethylolpropane, tris-hydroxymethyl aminomethane, etc.; andtetrahydric alcohols such as pentaerythrit, dipentaerythrit, etc.Examples of polyvalent carboxylic acids include dibasic acids such asphthalic anhydride, isophthalic acid, terephthalic acid, succinicanhydride, adipic acid, azelaic acid, sebacic acid, tetrahydrophthalicanhydride, hexahydrophthalic anhydride, tetrabromophthalic anhydride,tetrachlorophthalic anhydride, endomethylene tetrahydrophthalicanhydride, maleic anhydride, fumaric anhydride, itaconic acid, etc.;tribasic acids such as trimellitic anhydride,methylcyclohexenetricarboxylic acid, etc.; and tetrabasic acids such aspyromellitic anhydride. Examples of the aliphatic acids are caprylicacid, capric acid, lauric acid, myristic acid, palmitic acid, stearicacid, oleic acid, ricinoleic acid, linolenic acid, eleostearic acid, andso forth.

In some cases, the alkyd resins synthesized from the polyhydricalcohols, the polybasic acids and the aliphatic acids described aboveare used, but in many cases, they are used after being subjected tovarious modification treatments. Examples of the modified resins includephenol-alkyd resins obtained by adding phenols to the alkyd resin andhaving improved chemical resistance and adhesion to the steel material,bisphenol type epoxy resins, epoxy-modified alkyd resins obtained byadding alicyclic or aliphatic epoxy compounds (the resin of this typeare sometimes referred to as the “epoxypolyester resins”) and vinylalkydresins obtained by adding styrene, vinyltoluene, acrylic acid esters ormethacrylic acid esters. Curing of these various kinds of alkyd resinscan be made by a melamine resin or a urea resin. They can be cured byoxidation in air using organometallic salts such as an organic acid withlead, manganese, cobalt, etc. In the present invention, these alkydresins can be selected optionally in order to satisfy performance otherthan the cathodic disbonding resistance, and the applicability ofcoating.

The term “humidity curing urethane resin” means those resins which areprepared by synthesizing a resin having an isocyanate group left at theterminal thereof by reacting an isocyanate in excess with a polyol resinand a polyhydric alcohol and reacting and curing the isocyanate groupsbetween the resins by the moisture in air. Examples of the polyol resinsused here include a polyether polyol resin, a polyester polyol resin, anacrylic polyol resin, etc., and examples of the isocyanate compoundsinclude aromatic isocyanates such as toluene isocyanate, 4,4′-diphenylisocyanate, xylylene diisocyanate, isophorone diisocyanate, etc.;hexamethylene diisocyanate, saudine diisocyanate, various isocyanatesobtained by hydrogenating the aromatic isocyanates described above, andaliphatic isocyanates such as trimethylhexamethylene diisocyanate anddimer acid diisocyanates. In the present invention, the humidity curingurethane resin synthesized by arbitrarily selecting the polyol resin andvarious isocyanate compounds described above is used.

Further, the present invention may use a phenoxy resin. The phenoxyresin is an epoxy resin which is derived from bisphenol and has anextremely large molecular weight. The number average molecular weightthereof is at least 10,000 and the number of epoxy groups is extremelysmall. Examples of commercially available phenoxy resins are “DER684” (aproduct of Dow Chemical), “EPOTOHTO YD050” and “EPOTOHTO YD040”(products of Tohoto Kasei K.K.).

The ratio of the curing agent/reactive resin component of the coatingsolution is preferably (0.6-1.1)/1.0, more preferably (0.8-1.0)/1.0, interms of the equivalent ratio of the reactive group of the curing agentand the functional groups capable of reacting with the reactive group ofthe curing agent.

The coating composition of the present invention further comprises azinc borate compound. The zinc borate compound promotes disbondmentresistance by reducing conversion of the steel substrate. The effect ismost noticeable in long term severe exposure tests, such as 28 daycathodic disbondment tests at elevated temperatures (e.g. 80° C.), wherethe zinc borate compound can reduce disbanding by 50%. The zinc boratecompound is added at low levels below 5% by weight, based on totalsolids, preferably from about 0.5-4.75%, more preferably 0.5-4.0%, andeven more preferably 1.5-2.5%. By using such amounts, the disbondment issignificantly reduced. Further, use of these low levels of zinc boratecompounds provides a significant decrease in cost, as it is commonlyknown that such zinc compounds are more expensive than thermosettingresin systems. Also, issues of zinc compound solubility, over longperiods of time, are improved when low levels are used.

The zinc borate compound constituting the anticorrosive coatingcomposition of the present invention, may comprise an individual zincborate compound, or a mixture of two or more zinc borate compounds.Examples of such zinc borate compound include, but are not limited to,zinc methaborate [Zn(BO₂)₂], basic zinc borate [ZnB₄O₇·2ZnO] and zincborate [2ZnO·3B₂O₃-3.5H₂O]. Zinc borate is preferably used. Zinc boratecan be obtained by melting a mixed starting material of zinc oxide andboric acid or double-decomposing the aqueous solution of the mixedstarting material. A particularly useful zinc borate compound is“Borogard ZB fine,” [2ZnO·3B₂O₃·3.5H₂O], available from U.S. Borax,Incorporated.

The coating compositions of the present invention may additionallycomprise one or more components taken, for example, from the groupconsisting of pigments, dyes, fillers, flow control agents, dispersants,thixotropic agents, adhesion promoters, antioxidants, light stabilizersand curing catalysts. They may also include other known anticorrosionagents, for example anticorrosion pigments, such as phosphate- orcontaining pigments, metal oxide pigments, for example calcium oxide orcombined calcium oxide/silica pigments, or other organic or inorganiccorrosion inhibitors, for example salts of nitroisophthalic acid,phosphoric esters, technical-grade amines or substituted benzotriazoles.

The pigments are, for example, titanium dioxide, iron oxide, aluminumbronze or phthalocyanine blue.

Examples of fillers are talc, alumina, aluminum silicate, barytes, mica,and silica. The corrosion inhibitors can be applied to a supportmaterial. Pulverulent fillers or pigments are particularly suitable forthis purpose.

Flow control agents and thixotropic agents are based, for example, onmodified bentonites or silicas.

Preferably the coating composition of the present invention containsfrom 0 to 55% by weight, more preferably 5 to 30% by weight, based upontotal solids weight, of fillers, pigments, additives, or any mixturesthereof.

In a preferred embodiment, the curable coating composition is a powdercoating composition prepared by conventional techniques employed in thepowder coatings art. Typically, the components of the powder coatingformulation are thoroughly blended together via medium to high intensitymixing and then melt blended in an extruder. Melt blending is generallycarried out in the temperature range of between about 220° F. and 280°F. with careful control of the extruder temperature to minimize anycuring and gelation from taking place in the extruder. The extrudedcomposition, usually in sheet form after cooling, is broken into chipsand then ground in a mill to a powder and subsequently screened toachieve the desired powder particle size.

The aforesaid curable powder coating composition of the presentinvention exhibits superior adhesive properties, as demonstrated byhaving superior resistance to cathodic disbondment, over an extendedperiod, together with very rapid cure speeds. These properties provide apowder coating that can be readily applied, by typical application meansin the powder coating art, to rebars, pipelines and other metallicsubstrates, some of which may require cold working after being coated.The superior adhesive properties of this invention provide the abilityto adhere to even oily an scaly surfaces, such as those encountered onsteel strappings and other marginally clean metallic substrates.

The cure time/temperature range of the aforementioned powder coatingcomposition of this invention is found to be from about 60 seconds atabout 470° F. to about 180 seconds at about 400° F.

In a typical powder coating procedure, the metal substrate is preheatedto a temperature from about 400° F. to 490° F. Then, the powder coatingis applied by standard means, such as fluidized bed immersion,electrostatic spray application, and the like. The residual heat in thepreheated metal substrate provides enables the powder coating finish tomelt, flow and begin to cure to a continuous, anticorrosive, dry film.

The aforesaid powder coated metal substrate may then be introduced intohigh temperature ovens, such as convection, infrared, or combinationovens, to melt, flow out and further cure into a smooth hardened film.In commercial high speed coating lines, the melt flow and cure timeusually ranges between about 40 and 140 seconds at a peak substratetemperature ranging between about 400° F. and 490° F. Subsequently, thecoated substrate is conveyed to a water quench to lower the temperatureto between about 100° F. and 200° F.

Steel substrates are usually coated with an effective amount of powdercoating to produce a dry film thickness of between about 5 and 20 milsthick or greater.

As mentioned above, the composition of the present invention ispreferably directed to a particulate form powder coating composition.However, any coating used at a solid concentration from about 10 to 100%may be possible. In addition to the aforementioned powder coating, othercoating types may include a solvent based type, a water based type, andthe like.

The anticorrosive coating composition having the composition describedabove can be produced by the same method as the production methods ofordinary coating compositions. In the case of a liquid non-solventcoating composition, for example, a predetermined amount of the zincborate compound is added to the thermosetting resin, and the mixture issubjected to dispersion treatment using a roll mill, a dissolver, etc.In the case of an organic solvent type coating composition, the mixtureis subjected to dispersion treatment using a roll mill, a dissolver, anSG mill, a pot mill, etc. To prepare a powder coating composition, apredetermined amount of the zinc borate compound is added to thethermosetting resin, and the mixture is premixed, then heat-kneaded,cooled, and thereafter pulverized and classified.

The present invention likewise relates to a process for preparing acorrosion-resistant surface coating on a corrodable metal surface, whichcomprises treating this surface with the coating composition of thepresent invention.

The coating of the anticorrosive coating method according to the presentinvention is applied by the use of a brush, a roller, an airless spray,an air spray, a powder coating mechanism, etc., which is selectedsuitably in accordance with the form of the composition, in a customarymanner. A heavy duty protective film such as polyethylene lining, heavyduty protective urethane coating composition, epoxy resin coatingcomposition, and the like, and/or a finishing layer such as a coloringlayer may be applied to the surface of the coating film after it iscoated.

The present invention will now be further illustrated by a considerationof the following examples which are intended to be purely exemplary ofthe invention.

EXAMPLES Test Procedure

The following cathodic disbondment test procedure was used forgenerating data reported in the examples below. Steel panels (4×4×⅝″),which were blasted and rinsed with phosphoric acid were coated with14-18 mils of red fusion bond epoxy by pre-heating to 470° F. thendipping in a fluidized bed. After a post cure of 3 minutes, the panelswere water quenched. The test panels were prepared for cathodicdisbondment by drilling a 3 mm diameter hole in the center of each testpanel, and sealing a 3.5 in diameter cylinder onto the panel. Thecylinder was filled with 3% NaCl solution, a Platinum wire was immersedin the solution and the entire assembly was placed in an oven set at 80°C. A voltage of 1.5V was applied across the Platinum wire and the testpanel. After 28 days in the oven, the panels were tested for disbondmentby removing the solution and cylinder then making 8 radial cuts in thecoating away from the holiday. The panel was left for one hour to coolto room temperature then the coating was removed with a knife by workingaway from the holiday edge using a levering action. The disbondment fromthe center of the holiday to edge of the disbonded area was measured,then averaged. This method follows TransCanada Pipeline spec. TESCOATFBE Rev.0, which is based on CSA Z245.20-98.

Examples 1 to 5

Table 1 below illustrates the preparation of cathodic disbondmentresistant thermosetting epoxy powder coating compositions, of thepresent invention, having zinc borate levels varied progressively from4.9% down to 0%, that are suitable for fusion coating on rebars,pipelines, and other metallic substrates. For examples 1 to 5, the epoxycuring agent is an accelerated dicyandiamide type curing agent. Allamounts are given in percent by weight of total formulation weight.TABLE 1 Ingredient Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Epon ™ 2024 epoxy resin67.5 67.5 67.5 67.5 67.5 (Resolution Performance Products, LLC.)Epicure ™ P104 curing agent 1.7 1.7 1.7 1.7 1.7 (Resolution PerformanceProducts, LLC.) Zinc Borate (Borogard ™ ZB, 0 0.5 1.5 3 4.9 US Borax,Inc.) Nyad ™ M400 filler (NYCO 29.5 29 28 26.5 24.6 Minerals, Inc.)Bayferrox ™ 140 iron oxide 1 1 1 1 1 pigment (Bayer Corp.) Cabosil ™ M5(Cabot, Inc.) 0.3 0.3 0.3 0.3 0.3

The epoxy powder coating compositions listed in Table 1 were then coatedon steel panels, cured, and subjected to long term performance tests, asdescribed above. The cathodic disbondment test results are listed belowin Table 2, and FIG. 1 further illustrates the cathodic disbondmentresistance correlation with zinc borate concentration. As shown,significant disbondment resistance was observed at zinc borate levels ator below 4.9% by weight, based upon total formulation weight. TABLE 2Cathodic Disbondment Test Results Example 1 Example 2 Example 3 Example4 Example 5 32.9 mm 26.3 mm 20.8 mm 18.6 mm 15.8 mm

Example 6

Table 3 below illustrates the preparation of a cathodic disbondmentresistant thermosetting epoxy powder coating composition, of the presentinvention having zinc borate level of 3% and a phenolic type curingagent. All amounts are given in percent by weight of total formulationweight. TABLE 3 Ingredient Example 6 Epon ™ 2024 epoxy resin (ResolutionPerformance 55 Products, LLC.) Epikure ™ P202 phenolic epoxy curingagent 11 (Resolution Performance Products, LLC.) Zinc Borate (Borogard ™ZB, US Borax, Inc.) 3 Nyad ™ M400 filler (NYCO Minerals, Inc.) 29.7Bayferrox ™ 140 iron oxide pigment (Bayer Corp.) 1 Cabosil ™ M5 (Cabot,Inc.) 0.3

The epoxy powder coating composition of Example 6, listed in Table 3,was then coated on steel panels, cured, and subjected to long termperformance tests, as described above. The resultant coating yielded a20.8 mm disbondment measurement, over the 28 day, 80° C., long termdisbondment test.

Various modifications, alterations, additions or substitutions of thisinvention will be apparent to those skilled in the art without departingfrom the spirit and scope of this invention. This invention is notlimited by the illustrative embodiments set forth herein, but rather isdefined by the following claims.

1. A curable coating composition comprising: (a) a thermosetting resin,or mixture of thermosetting resins; (b) from about 0.5 to 4.75% byweight, based upon total solids weight, of a zinc borate compound; and(c) a curing agent, or mixture of curing agents, capable of curing saidcoating composition in an amount effective to cure the coating.
 2. Thecoating composition of claim 1 wherein said composition is a powdercoating composition.
 3. The coating composition of claim 1 wherein saidthermosetting resin is at least one resin selected from (a) epoxy resinsand epoxy curing agents, (b) polyol resins and isocyanates, (c) acrylmodified epoxy resins and polymerization initiators, (d) alkyd resins,(e) humidity curing type urethane resins, and (f) phenoxy resins.
 4. Thecoating composition of claim 1 wherein said thermosetting resin is abisphenol A/epichlorohydrin epoxy functional resin.
 5. The coatingcomposition of claim 1 which comprises from about 25 to 90% by weight,based upon total solids weight, of said thermosetting resin, or mixturesthereof.
 6. The coating composition of claim 1 which comprises fromabout 60 to 80% by weight, based upon total solids weight, of saidthermosetting resin, or mixtures thereof.
 7. The coating composition ofclaim 1 which comprises from about 1.5 to 2.5% by weight, based upontotal solids weight, of said zinc borate compound.
 8. The coatingcomposition of claim 1 wherein said zinc borate compound is at least onecompound selected from the group consisting of zinc methaborate, basiczinc borate and zinc borate.
 9. The coating composition of claim 1wherein said zinc borate compound is zinc borate, as represented by theformula:2ZnO·3B₂O₃·3.5H₂O
 10. The coating composition of claim 1 wherein thesaid curing agent is an epoxy curing compound selected from the groupconsisting of aliphatic diamines, aromatic diamines, heterocyclicdiamines and the modified products of the above-mentioned diamines,polyamide resins reacted with aliphatic acids or the dimers thereof,acid anhydrides, thiols, phenols, dicyandiamide derivatives having anaddition reactivity and self-polyaddition catalytic activity, andimidazoles.
 11. The coating composition of claim 1 wherein said curingagent is an accelerated dicyandiamide functional epoxy curing compound.12. The coating composition of claim 1 wherein said curing agent is aphenolic functional epoxy curing compound.
 13. The coating compositionof claim 1 which comprises from about 1 to 35% by weight, based upontotal solids weight, of said curing agent, or mixtures thereof.
 14. Thecoating composition of claim 1 which comprises from about 2 to 20% byweight, based upon total solids weight, of said curing agent, ormixtures thereof.
 15. The coating composition of claim 1 which furthercomprises a calcium oxide filler, silicate filler, or mixture thereof.16. The coating composition of claim 1 which further comprises a pigmentselected from the group consisting of titanium dioxide, iron oxide,aluminum, bronze or phthalocyanine blue.
 17. The coating composition ofclaim 1 which further comprises an additive, or mixture of additivesselected from the group consisting of dyes, flow control agents,dispersants, thixotropic agents, adhesion promoters, antioxidants, lightstabilizers, curing catalysts, anticorrosion agents, and substitutedbenzotriazoles.
 18. The coating composition of claim 1 which comprisesfrom about 0 to 55% by weight, based upon total solids weight, offillers, pigments, additives, or any mixtures thereof.
 19. The coatingcomposition of claim 1 which comprises from about 20 to 35% by weight,based upon total solids weight, of fillers, pigments, additives, or anymixtures thereof.
 20. An epoxy powder coating composition comprising athermosetting epoxy resin and curing agent for said epoxy resin, whereinthe improvement is the addition of from about 0.5 to 4.75% by weight,based upon total solids weight, of a zinc borate compound for improvedresistance to cathodic disbondment.
 21. A method of cathode protectionfor a steel material, comprising the steps of: (a) subjecting the steelsurface to a mechanical treatment, (b) applying to said treated steelsurface, the coating composition of claim 1, and (c) polarizing thecoated steel material as a cathode.
 22. The method of cathode protectionclaimed in claim 21 wherein a heavy duty protective film and/or afinishing layer is further applied to the surface of the coated film.